Characteristic size of molecular dynamics in polymers probed by dielectric probes of variable length

Abstract

Recently we introduced a rigid-rod type chromophore, E-4,4 0 -N,N-dibutylamino-nitro-stilbene (DBANS), that acts as a dielectric and fluorescent probe, and enables the study of molecular dynamics in apolar polymers by dielectric relaxation spectroscopy (DRS). This work focuses on the effect of the probe size and concentration on the specific probe response. For this purpose, a series of probe molecules with core lengths (l) between 0.6 and 2 nm was used to ÔdopeÕ polystyrene at concentrations from 0.1 to 1.0 wt%. For all probes, a selective amplification of the intrinsic a-relaxation was found that was linear within the whole probe concentration range. Moreover, two clear size effects on the probe dynamics were revealed. At high temperatures, the probe relaxation time obeys s / l 3 , which confirmed rotational diffusion as the linking mechanism between the polymeric segmental dynamics and dielectric probe response. The shortest probe revealed an additional Arrhenius-type (b * ) relaxation that was attributed to large angular probe fluctuations involving discrete jumps. Its exclusive occurrence indicates a critical lower probe size (l * $ 0.6 nm) below which the dielectric probe acts also as a Ôfree-volume probeÕ as indicated by the pronounced anormality in the s b à ðT Þ dependence. Finally, an attempt was made to relate probe concentration effects on the glass transition temperature (plasticizing effect) to the intrinsic length scale of cooperative dynamics n. Based on a simple model for local plasticization we have determined a lower bound for n in the order of 5 nm.

Effect of temperature and pressure on the structural (α-) and the true Johari–Goldstein (β-) relaxation in binary mixtures

“…It is generally agreed that s b has an Arrhenius temperature dependence in the glassy state but recently some deviations, more than the fit uncertainty, from this behavior were shown when the glass transition was crossed [8,16]. In such cases, as well as in our case (see Fig.…”

Probing the cooperative dynamics varying the side-chain length of poly(alkyl acrylate)s: ESR experiments

“…This represents one of the reasons of the appeal exerted by molecular probe spectroscopy, as its recent application in the framework of dielectric spectroscopy also testifies [10]. With time resolved fluorescence measurements of doped malachite green molecules dissolved in different glass formers [11] and with positron annihilation lifetime spectroscopy studies [12] dynamic anomalies were enlightened in the range 1.15-1.4T g .…”

Dielectric relaxation of various end-functionalized polystyrenes: Plastification effects versus specific dynamics

“…In this way, by selectively amplifying the dielectric signal, it was envisioned that it might be possible to determine relationships and correlations of individual fluctuations contributing to the overall chain dynamics by 'labeling' a polymer with covalently bound polar groups in welldefined positions along the chain. Such an approach offers clear advantages over classical experiments usually involving mixing small molecules as probes into a polymer matrix and measuring the effective local response by some means [3][4][5]. First, this latter approach gives a less direct way of evaluating the polymer dynamics simply because the probe is not necessarily directly 'slaved' to the chain dynamics.…”

“…First, this latter approach gives a less direct way of evaluating the polymer dynamics simply because the probe is not necessarily directly 'slaved' to the chain dynamics. Second, as the second probe molecule often is of very different nature from the polymer matrix, the mixing may result in more severe thermodynamic perturbations that may change or interfere with the dynamics of the polymer [6] leading to plastification effects [5] or even phase separation [7]. However, on the other hand, the required polymer functionalization is not trivial -first, it requires complete control of the polymer synthesis such that the functional groups are located on well-defined and pre-determined places in the chain.…”